The typical motor vehicle utilizes electronic fuel injection (EFI) to deliver fuel into the engine. The fuel injectors (solenoid valves) are electronically connected to an engine control module that controls the amount of fuel entering the engine via control of the solenoid valves. By changing the dwell time of the valves, the amount of fuel entering the engine can be controlled. Fluctuations in engine performance and operating conditions can affect fuel pressure in the fuel system and hence the amount of fuel entering the engine. There are essentially two types of EFI systems, return-style and returnless, that are utilized to control fuel pressure. Typical return-style EFI systems rely on mechanical means to control fuel system delivery pressure by utilizing a return line from a fuel pressure regulator. A returnless system must rely upon electronic means for fuel pressure control. In this regard, the typical returnless system regulates fuel pressure by means of a fuel rail pressure sensor connected to electronics that can control fuel pump speed.
FIG. 1 depicts a return-style fuel system that is well known in the prior art. As shown in FIG. 1, fuel system 1 for an engine-driven vehicle having EFI includes a fuel tank 2, a fuel pump 3 and a fuel line 4 that delivers fuel from pump 3 to fuel injectors 5 disposed in fuel rail 6. Fuel line 4 includes fuel filter 7 and check valve 8. Fuel injectors (solenoid valves) 5 are mounted inside rail 6 and deliver fuel into engine intake manifold 10 carried by the engine 11. In a typical engine layout, nozzles (not shown) of the individual fuel injectors 5 are positioned adjacent to the fuel/air intake ports of the associated cylinders (not shown) of the engine 11.
In a return-style fuel system, line 9 connects fuel rail 6 to a bypass-style fuel pressure regulator 12, which is in turn connected to return line 13 leading back to fuel tank 2. Fuel pump 3 of the typical return-style EFI fuel system is electrically driven and operates at a continuous (constant-speed) high flow rate while the bypass style fuel pressure regulator 12 returns unused fuel back to the tank. The engine management electronics can adjust dwell time of the fuel injectors 5 in response to a variety of engine operating conditions such as intake manifold pressure, throttle position, engine speed or oxygen level. Typically the engine management electronics do not modulate dwell time based upon fuel pressure proper. Hence, in a conventional return-style fuel system, fuel pressure is assumed to be at a proper level in the fuel rail 6 from the standpoint of setting fuel injector dwell times. The advantages of this fuel system include its simple operation and low cost, along with generally consistent fuel pressure that responds rapidly to sudden changes in demand for fuel flow to the engine.
The prior art fuel pressure regulator 12 operates to return over-pressurized, excess fuel to the tank. In this regard, fuel pressure regulator 12 acts like a gate and allows fuel to return to the tank only when a calibrated fuel rail pressure is reached. When this calibrated fuel pressure is reached, excess fuel will be permitted to return to the tank and fuel pressure in the fuel rail will be maintained. An example prior art fuel pressure regulator is depicted in FIG. 2. The prior art fuel pressure regulator includes an air chamber 17 and a fill chamber 14 that are separated from each other by a diaphragm 15. Air chamber 17 is plumbed to the engine intake manifold via vacuum line 25. Fill chamber 14 is fluidly connected to the fuel rail 6 via line 9. Fill chamber 14 and air chamber 17 are on opposite sides of diaphragm 15. The fuel pressure regulator adjusts fuel pressure of the fill chamber 14 (fuel pressure applied to the fuel injector valves) to be higher than manifold negative pressure acting on the air chamber 17 by a predetermined a degree (for example 2.5 atmosphere). In working operation, movement (expansion) of the diaphragm is opposed by the force of spring 18. Spring 18 biases diaphragm 15, which has an integral valve 16 on valve seat 19. For simplicity of explanation, when a difference between fuel pressure and manifold negative pressure becomes larger than a predetermined value, diaphragm 15 is forced up. Integral valve 16 moves in cooperation with diaphragm 15. As a result of the lifting of the valve, an opening degree of a throttle portion made up of the movable valve 16 and valve seat 19 becomes large enough to allow excess fuel to enter return passage 20 and flow back into the tank. By regulating fuel pressure in this fashion the prior art fuel pressure regulator maintains fuel pressure in fill chamber 14 at a constant pressure. This type of bypass style regulator is common on return-style fuel injection systems to allow change in fuel pressure as a function of intake manifold pressure.
Disadvantages of this system include a relatively high current draw in the system leading to higher fuel temperatures, particularly in high flow applications. Another disadvantage occurs in a fuel system having a constant speed pump. In such a system the electric fuel pump operates at a constant speed above maximum engine demand. This action requires the maximum operating current to the fuel pump during all engineered fuel demand operating conditions. During extended periods of fuel pump operation, operating temperatures can get high enough to cause fuel pump cavitation and pump failure. High flow fuel systems develop even higher current draw and demand for higher current levels.
Further disadvantages of this type of system include the limited ability to have the fuel pump speed effectively engage as a function of engine demand without the use of electronic control. Additionally, in this type of system, changes in fuel pressure result when the speed of the fuel pump changes due to fuel pressure regulator performance (regulation slope). Also, these systems when employed with bypass style regulators exhibit certain undesirable features. For example, these systems typically rely on the vehicle operator to manually set pump speed when operating at low speed, then increase speed during high engine demand.
FIG. 3 depicts a returnless fuel system 40. A returnless fuel system lacks regulator 12 and return line 13 and relies upon fuel pump modulation to control fuel pressures in the fuel rail. The prior art returnless fuel system uses a pressure transducer 22 measuring fuel rail pressure connected to an ECM 21. ECM 21 may also differentially measure fuel rail pressure against intake manifold pressure via sensor 23. ECM 21 is electrically connected to fuel pump 3. In response to an input from the pressure transducer, ECM 21 can lower or raise the fuel pump speed (typically via pulse width modulation) to maintain constant pressure in the fuel rail as a function of engine demand. Advantages of this system include weight and cost savings due to the absence of the regulator and return line. Also, with this system the fuel pump draws less current. Less current draw during low engine demand improves efficiency and results in less heat in the overall fuel system, though in some cases fuel in the fuel rails is allowed to heat up during low engine demands.
The prior art returnless fuel system has certain disadvantages. Disadvantages include slower system reaction time in responding to sudden changes in engine flow demand. Additionally, this system requires an accumulator to dampen fuel pressure spikes. Also, the fuel pump of the returnless fuel system is designed to operate at lower power conditions during low engine demand. However, for high flow fuel systems, reaction time of returnless fuel systems can be disadvantageously limiting. During long periods of low engine demand, fuel temperatures in the fuel rail can also be inconsistent by not using a return line.
High power (high flow) fuel systems have particularly troublesome heat build-up problems. High current draw during idle and low cruise put extra strain on the vehicle charging system as well. To address these problems, electronic speed controllers are used to reduce the speed of the pump during low engine demand operating conditions. These systems, however, typically require the inconvenience of the vehicle operator having to manually set pump speed when operating at low speed, then increase speed during high engine demand.